Well here is a situation.
I create 3 templates and each one of them will have a MasterPage. Eg. TemplateMaster1, TemplateMaster2 and TemplateMaster3
Then I create another MasterPage called SiteMasterPage
and in SiteMaster I call one of the three templates ( eg. TemplateMaster1)
Then I create a default.aspx page which will refer to the SiteMasterPage
Now the <asp:ContentPlaceHolder> tags which are in TemplateMaster1 can not be directly used in my default.aspx page
In order to use them here is a work around
Whereever you want to put the custom content.
Create a <asp:Content> tag in your SiteMasterPage first and then within it create a <asp:ContentPlaceHolder> tag
Here is an example.
<asp:Content ID=”Main” ContentPlaceHolderID=”TemplateMain” runat=”server”>
<asp:ContentPlaceHolder ID=”MasterMain” runat=”server”>
</asp:ContentPlaceHolder>
</asp:Content>
Then in your default.aspx page you can place the MasterMain
<asp:Content Id=”Main” ContentPlaceHolderID =MasterMain runat=”server”>
<h1>Welcome to this content which is using nested masterpages</h1>
</asp:Content>